Diaphragm carburetors are commonly used to provide the fuel requirements for two and four cycle internal combustion engines such as those typically found in hand-operated fuel-powered devices such as chainsaws, weed cutters/trimmers, lawn mowers and the like. A choke valve is often incorporated in the diaphragm carburetor when the carburetor is used in hand-operated devices having engines that operate under “cold start” conditions. The choke valve is located within a fuel and air mixing passage between a venturi and an inlet for air. The choke valve generally includes a plate that can be rotated between closed and opened positions as known in the art.
In some diaphragm carburetors, air flows into the fuel and air mixing passage and into an air bleed passage that connects with a fuel chamber to provide additional air to be mixed with the fuel thereby providing a leaner fuel and air mixture. A separate air bleed shut-off valve has been provided to selectively prevent communication between the air bleed passage and the fuel chamber so that a richer fuel and air mixture is delivered to the engine, for example, to facilitate starting and warming up a cold engine. The air bleed shut-off valve adds components, complexity and cost to the carburetor in that they must be mounted in the air bleed passage, and can require separate actuation that complicates an engine starting procedure.